Continuous ink jet printing systems operate by continuously discharging a stream of pressurized ink through a nozzle toward a substrate to be marked. The nozzle is coupled to a piezoelectric transducer or the like which is vibrated with a sinusoidal waveform at a frequency that causes the stream of ink to break off into substantially uniform drops shortly after being discharged from the nozzle.
Upon breakoff, each of the drops is subsequently passed through a selectively variable electric field associated with a charging electrode which selectively charges the drop. The amount of charge received by each drop is ordinarily controlled by adjusting the level of a voltage on the charging electrode that generates the electric field. Thereafter, an electric field generated by deflection plates deflect the drop according to the charge thereon. By appropriately varying the charging voltage and synchronizing it with the formation of each drop according to the amount of deflection desired therefor, drops are selectively deflected to form characters or other images on a moving target substrate. Drops that are not used for character or image formation are substantially uncharged and intercepted by a catcher for recirculation through the system. Two such systems are described in U.S. Pat. Nos. 3,683,396 and 3,972,474, and have been assigned to the same assignee of the present invention.
During the formation of a drop, the drop remains temporarily connected to the stream by a thin filament of ink. Eventually the drop and filament separate from each other and from the stream, whereby the filament may form its own, smaller drop known as a satellite.
If the satellite has a speed that is greater than that of its associated primary drop, it is known as a fast satellite. Conversely, if the satellite has a speed that is slower than that of its primary drop, it is known as a slow satellite. Factors in determining how the drops and satellites will break off from the stream include the frequency and amplitude of the driving signal, the physical properties of the ink, and the geometric characteristics of the nozzle.
A fast satellite catches up to and recombines with its primary drop, while a slow satellite is caught by and combines with the next subsequently-formed primary drop that trails it. Since each satellite may be charged with charge that was removed from its associated primary drop, fast satellites recombine with the primary drop without adversely affecting the charge-dependent amount of deflection of the primary drop. However, a slow satellite may alter the desired amount of charge on the subsequent drop. This results in an unintended amount of charge on either the primary drop or the subsequent drop, or on both drops, and therefore results in an unintended amount of deflection of the drops, thereby adversely affecting the quality of the resultant image. Thus, typical continuous ink jet printers are arranged to suppress satellite formation as much as possible, or at least to produce fast satellites in a manner that does not degrade the resultant image. This is ordinarily accomplished by increasing the amplitude of a sinusoidal driving waveform producing the nozzle vibration until satellite formation suitable for desirable image quality is achieved.
A condition wherein no more than three fast satellites are present in the drop stream (i.e., the third primary drop from the nozzle and its corresponding fast satellite have recombined before a new satellite is formed near the breakoff point with the next primary drop) has been found to be an acceptable condition for many printing operations. Accordingly, it is often desirable to arrange the system and the parameters influencing the breakoff characteristics so that no more than three fast satellites are produced in the drop stream, a printing condition known as a "three fast satellite" condition.
However, with certain inks and/or nozzles, desirable satellite conditions cannot be consistently achieved using conventional methods of breaking up an ink stream. While increasing the amplitude of the excitation signal producing the vibration to some extent desirably regulates satellite formation in some ink and nozzle combinations, other ink and nozzle combinations are unable to achieve acceptable satellite conditions, or require increases in driving amplitude that exceed the power driving capabilities of currently existing nozzle drive circuitry. For example, even at very large amplitudes, sinusoidal waveforms cannot achieve a fast satellite condition suitable for desirable image quality with certain inks.
In particular, continuous ink jet printing with hot-melt inks poses a substantial difficulty. Hot-melt inks exist in a solid phase at room temperature and are heated to a liquid phase for discharging. Satellite formation difficulties arise primarily as a result of the relatively low surface tension and high viscosity of hot-melt inks.
For example, typical liquid inks have a viscosity of 2 centipoise, a surface tension of 40 millinewtons per meter and a density of 1000 kilograms per cubic meter, versus a typical hot-melt ink viscosity of 10 centipoise, a surface tension of 18 millinewtons per meter and a density of 950 kilograms per cubic meter.
As a result, even large increases in driving amplitude have been found incapable of adequately breaking off hot-melt ink drops to form desired satellite conditions. Nevertheless, despite the drawbacks, continuous ink jet printing with hot melt inks is desirable to the industry because hot-melt inks have faster drying times compared to liquid inks. In addition, hot-melt inks substantially do not contain environmentally harmful volatile organic compounds.